Octal

Numeral systems,bitsandGray code

hex	dec	oct	3	2	1	0	step
0hex	00dec	00oct	0	0	0	0	g0
1hex	01dec	01oct	0	0	0	1	h1
2hex	02dec	02oct	0	0	1	0	j3
3hex	03dec	03oct	0	0	1	1	i2
4hex	04dec	04oct	0	1	0	0	n7
5hex	05dec	05oct	0	1	0	1	m6
6hex	06dec	06oct	0	1	1	0	k4
7hex	07dec	07oct	0	1	1	1	l5
8hex	08dec	10oct	1	0	0	0	vF
9hex	09dec	11oct	1	0	0	1	uE
Ahex	10dec	12oct	1	0	1	0	sC
Bhex	11dec	13oct	1	0	1	1	tD
Chex	12dec	14oct	1	1	0	0	o8
Dhex	13dec	15oct	1	1	0	1	p9
Ehex	14dec	16oct	1	1	1	0	rB
Fhex	15dec	17oct	1	1	1	1	qA

Octal(base 8) is anumeral systemwitheightas thebase.

In the decimal system, each place is apower of ten.For example:

$\mathbf {74} _{10}=\mathbf {7} \times 10^{1}+\mathbf {4} \times 10^{0}$

In the octal system, each place is a power of eight. For example:

$\mathbf {112} _{8}=\mathbf {1} \times 8^{2}+\mathbf {1} \times 8^{1}+\mathbf {2} \times 8^{0}$

By performing the calculation above in the familiar decimal system, we see why 112 in octal is equal to$64+8+2=74$in decimal.

Octal numerals can be easily converted frombinaryrepresentations (similar to aquaternary numeral system) by grouping consecutive binary digits into groups of three (starting from the right, for integers). For example, the binary representation for decimal 74 is 1001010. Two zeroes can be added at the left:(00)1 001 010,corresponding to the octal digits1 1 2,yielding the octal representation 112.

The octalmultiplication table

×	1	2	3	4	5	6	7	10
1	1	2	3	4	5	6	7	10
2	2	4	6	10	12	14	16	20
3	3	6	11	14	17	22	25	30
4	4	10	14	20	24	30	34	40
5	5	12	17	24	31	36	43	50
6	6	14	22	30	36	44	52	60
7	7	16	25	34	43	52	61	70
10	10	20	30	40	50	60	70	100

Usage

In China

Fuxi's "Earlier Heaven" Arrangement of the Eight Trigrams

The eightbaguaor trigrams of theI Chingcorrespond to octal digits:

• 0 = ☷, 1 = ☳, 2 = ☵, 3 = ☱,
• 4 = ☶, 5 = ☲, 6 = ☴, 7 = ☰.

Gottfried Wilhelm Leibnizmade the connection between trigrams, hexagrams and binary numbers in 1703.

By Native Americans

• TheYuki languageinCaliforniahas an octal system because the speakers count using the spaces between their fingers rather than the fingers themselves.
• ThePamean languagesinMexicoalso have an octal system, because their speakers count on the knuckles of a closed fist.

By Europeans

• It has been suggested that the reconstructedProto-Indo-European (PIE)word for "nine" might be related to the PIE word for "new". Based on this, some have speculated that proto-Indo-Europeans used an octal number system, though the evidence supporting this is slim.
• In 1668,John WilkinsinAn Essay towards a Real Character, and a Philosophical Languageproposed use of base 8 instead of 10 "because the way of Dichotomy or Bipartition being the most natural and easie kind of Division, that Number is capable of this down to an Unite".
• In 1716, KingCharles XII of SwedenaskedEmanuel Swedenborgto elaborate a number system based on 64 instead of 10. Swedenborg argued, however, that for people with less intelligence than the king such a big base would be too difficult and instead proposed 8 as the base. In 1718 Swedenborg wrote (but did not publish) a manuscript: "En ny rekenkonst som om vexlas wid Thalet 8 i stelle then wanliga wid Thalet 10"(" A new arithmetic (or art of counting) which changes at the Number 8 instead of the usual at the Number 10 "). The numbers 1–7 are there denoted by the consonants l, s, n, m, t, f, u (v) and zero by the vowel o. Thus 8 =" lo ", 16 =" so ", 24 =" no ", 64 =" loo ", 512 =" looo "etc. Numbers with consecutive consonants are pronounced with vowel sounds between in accordance with a special rule.
• Writing under the pseudonym "Hirossa Ap-Iccim" inThe Gentleman's Magazine,(London) July 1745,Hugh Jonesproposed an octal system for British coins, weights and measures. "Whereas reason and convenience indicate to us an uniform standard for all quantities; which I shall call theGeorgian standard;and that is only to divide every integer in eachspeciesinto eight equal parts, and every part again into 8 real or imaginary particles, as far as is necessary. For tho' all nations count universally bytens(originally occasioned by the number of digits on both hands) yet 8 is a far more complete and commodious number; since it is divisible into halves, quarters, and half quarters (or units) without a fraction, of which subdivisiontenis uncapable.... "In a later treatise onOctave computation(1753) Jones concluded: "Arithmetic byOctavesseems most agreeable to the Nature of Things, and therefore may be called Natural Arithmetic in Opposition to that now in Use, by Decades; which may be esteemed Artificial Arithmetic. "
• In 1801,James Andersoncriticized the French for basing themetric systemon decimal arithmetic. He suggested base 8, for which he coined the termoctal.His work was intended as recreational mathematics, but he suggested a purely octal system of weights and measures and observed that the existing system ofEnglish unitswas already, to a remarkable extent, an octal system.
• In the mid-19th century, Alfred B. Taylor concluded that "Our octonary [base 8]radixis, therefore, beyond all comparison the "best possible one"for an arithmetical system." The proposal included a graphical notation for the digits and new names for the numbers, suggesting that we should count "un,du,the,fo,pa,se,ki,unty,unty-un,unty-du"and so on, with successive multiples of eight named"unty,duty,thety,foty,paty,sety,kityandunder."So, for example, the number 65 (101 in octal) would be spoken in octonary asunder-un.Taylor also republished some of Swedenborg's work on octal as an appendix to the above-cited publications.

In computers

Octal became widely used in computing when systems such as theUNIVAC 1050,PDP-8,ICL 1900andIBM mainframesemployed6-bit,12-bit,24-bitor36-bitwords. Octal was an ideal abbreviation of binary for these machines because their word size is divisible by three (each octal digit represents three binary digits). So two, four, eight or twelve digits could concisely display an entiremachine word.It also cut costs by allowingNixie tubes,seven-segment displays,andcalculatorsto be used for the operator consoles, where binary displays were too complex to use, decimal displays needed complex hardware to convert radices, andhexadecimaldisplays needed to display more numerals.

All modern computing platforms, however, use 16-, 32-, or 64-bit words, further divided intoeight-bit bytes.On such systems three octal digits per byte would be required, with the most significant octal digit representing two binary digits (plus one bit of the next significant byte, if any). Octal representation of a 16-bit word requires 6 digits, but the most significant octal digit represents (quite inelegantly) only one bit (0 or 1). This representation offers no way to easily read the most significant byte, because it's smeared over four octal digits. Therefore, hexadecimal is more commonly used in programming languages today, since two hexadecimal digits exactly specify one byte. Some platforms with a power-of-two word size still have instruction subwords that are more easily understood if displayed in octal; this includes thePDP-11andMotorola 68000 family.The modern-day ubiquitousx86 architecturebelongs to this category as well, but octal is rarely used on this platform, although certain properties of the binary encoding of opcodes become more readily apparent when displayed in octal, e.g. the ModRM byte, which is divided into fields of 2, 3, and 3 bits, so octal can be useful in describing these encodings. Before the availability ofassemblers,some programmers would handcode programs in octal; for instance, Dick Whipple and John Arnold wroteTiny BASIC Extendeddirectly in machine code, using octal.

Octal is sometimes used in computing instead of hexadecimal, perhaps most often in modern times in conjunction withfile permissionsunderUnixsystems (seechmod). It has the advantage of not requiring any extra symbols as digits (the hexadecimal system is base-16 and therefore needs six additional symbols beyond 0–9).

In programming languages, octalliteralsare typically identified with a variety ofprefixes,including the digit0,the lettersoorq,the digit–letter combination0o,or the symbol&or$.InMotorola convention,octal numbers are prefixed with@,whereas a small (or capital) letteroorqis added as apostfixfollowing theIntel convention.InConcurrent DOS,Multiuser DOSandREAL/32as well as inDOS PlusandDR-DOSvariousenvironment variableslike$CLS,$ON,$OFF,$HEADERor$FOOTERsupport an\nnnoctal number notation,and DR-DOSDEBUGutilizes\to prefix octal numbers as well.

For example, the literal 73 (base 8) might be represented as073,o73,q73,0o73,\73,@73,&73,$73or73oin various languages.

Newer languages have been abandoning the prefix0,as decimal numbers are often represented with leading zeroes. The prefixqwas introduced to avoid the prefixobeing mistaken for a zero, while the prefix0owas introduced to avoid starting a numerical literal with an Alpha betic character (likeoorq), since these might cause the literal to be confused with a variable name. The prefix0oalso follows the model set by the prefix0xused for hexadecimal literals in theC language;it is supported byHaskell,OCaml,Pythonas of version 3.0,Raku,Ruby,Tclas of version 9,PHPas of version 8.1,RustandECMAScriptas of ECMAScript 6(the prefix0originally stood for base 8 inJavaScriptbut could cause confusion,therefore it has been discouraged in ECMAScript 3 and dropped in ECMAScript 5).

Octal numbers that are used in some programming languages (C,Perl,PostScript...) for textual/graphical representations of byte strings when some byte values (unrepresented in a code page, non-graphical, having special meaning in current context or otherwise undesired) have to be toescapedas\nnn.Octal representation may be particularly handy with non-ASCII bytes ofUTF-8,which encodes groups of 6 bits, and where any start byte has octal value\3nnand any continuation byte has octal value\2nn.

Octal was also used forfloating pointin theFerranti Atlas(1962),Burroughs B5500(1964),Burroughs B5700(1971),Burroughs B6700(1971) andBurroughs B7700(1972) computers.

In aviation

Transpondersin aircraft transmit a "squawk"code,expressed as a four-octal-digit number, when interrogated by ground radar. This code is used to distinguish different aircraft on the radar screen.

Conversion between bases

Decimal to octal conversion

Method of successive Euclidean division by 8

To convert integer decimals to octal,dividethe original number by the largest possible power of 8 and divide the remainders by successively smaller powers of 8 until the power is 1. The octal representation is formed by the quotients, written in the order generated by the algorithm. For example, to convert 125₁₀to octal:

125 = 8²×1+ 61

61 = 8¹×7+ 5

5 = 8⁰×5+ 0

Therefore, 125₁₀= 175₈.

Another example:

900 = 8³×1+ 388

388 = 8²×6+ 4

4 = 8¹×0+ 4

4 = 8⁰×4+ 0

Therefore, 900₁₀= 1604₈.

Method of successive multiplication by 8

To convert a decimal fraction to octal, multiply by 8; the integer part of the result is the first digit of the octal fraction. Repeat the process with the fractional part of the result, until it is null or within acceptable error bounds.

Example: Convert 0.1640625 to octal:

0.1640625 × 8 = 1.3125 =1+ 0.3125

0.3125 × 8 = 2.5 =2+ 0.5

0.5 × 8 = 4.0 =4+ 0

Therefore, 0.1640625₁₀= 0.124₈.

These two methods can be combined to handle decimal numbers with both integer and fractional parts, using the first on the integer part and the second on the fractional part.

Method of successive duplication

To convert integer decimals to octal, prefix the number with "0.". Perform the following steps for as long as digits remain on the right side of the radix: Double the value to the left side of the radix, usingoctalrules, move the radix point one digit rightward, and then place the doubled value underneath the current value so that the radix points align. If the moved radix point crosses over a digit that is 8 or 9, convert it to 0 or 1 and add the carry to the next leftward digit of the current value.Addoctallythose digits to the left of the radix and simply drop down those digits to the right, without modification.

Example:

0.4 9 1 8 decimal value
+0
---------
4.9 1 8
+1 0
--------
6 1.1 8
+1 4 2
--------
7 5 3.8
+1 7 2 6
--------
1 1 4 6 6. octal value

Octal to decimal conversion

To convert a numberkto decimal, use the formula that defines its base-8 representation:

$k=\sum _{i=0}^{n}\left(a_{i}\times 8^{i}\right)$

In this formula,a_iis an individual octal digit being converted, whereiis the position of the digit (counting from 0 for the right-most digit).

Example: Convert 764₈to decimal:

764₈= 7 × 8²+ 6 × 8¹+ 4 × 8⁰= 448 + 48 + 4 = 500₁₀

For double-digit octal numbers this method amounts to multiplying the lead digit by 8 and adding the second digit to get the total.

Example: 65₈= 6 × 8 + 5 = 53₁₀

Method of successive duplication

To convert octals to decimals, prefix the number with "0.". Perform the following steps for as long as digits remain on the right side of the radix: Double the value to the left side of the radix, usingdecimalrules, move the radix point one digit rightward, and then place the doubled value underneath the current value so that the radix points align.Subtractdecimallythose digits to the left of the radix and simply drop down those digits to the right, without modification.

Example:

0.1 1 4 6 6 octal value
-0
-----------
1.1 4 6 6
- 2
----------
9.4 6 6
- 1 8
----------
7 6.6 6
- 1 5 2
----------
6 1 4.6
- 1 2 2 8
----------
4 9 1 8. decimal value

Octal to binary conversion

To convert octal to binary, replace each octal digit by its binary representation.

Example: Convert 51₈to binary:

5₈= 101₂

1₈= 001₂

Therefore, 51₈= 101 001₂.

Binary to octal conversion

The process is the reverse of the previous algorithm. The binary digits are grouped by threes, starting from the least significant bit and proceeding to the left and to the right. Add leading zeroes (or trailing zeroes to the right of decimal point) to fill out the last group of three if necessary. Then replace each trio with the equivalent octal digit.

For instance, convert binary 1010111100 to octal:

001	010	111	100
1	2	7	4

Therefore, 1010111100₂= 1274₈.

Convert binary 11100.01001 to octal:

011	100	.	010	010
3	4	.	2	2

Therefore, 11100.01001₂= 34.22₈.

Octal to hexadecimal conversion

The conversion is made in two steps using binary as an intermediate base. Octal is converted to binary and then binary to hexadecimal, grouping digits by fours, which correspond each to a hexadecimal digit.

For instance, convert octal 1057 to hexadecimal:

To binary:

1	0	5	7
001	000	101	111

then to hexadecimal:

0010	0010	1111
2	2	F

Therefore, 1057₈= 22F₁₆.

Hexadecimal to octal conversion

Hexadecimal to octal conversion proceeds by first converting the hexadecimal digits to 4-bit binary values, then regrouping the binary bits into 3-bit octal digits.

For example, to convert 3FA5₁₆:

To binary:

3	F	A	5
0011	1111	1010	0101

then to octal:

0	011	111	110	100	101
0	3	7	6	4	5

Therefore, 3FA5₁₆= 37645₈.

Real numbers

Fractions

Due to having only factors of two, many octal fractions have repeating digits, although these tend to be fairly simple:

Decimal base Prime factors of the base:2,5 Prime factors of one below the base:3 Prime factors of one above the base:11 Other Prime factors:7 13 17 19 23 29 31			Octal base Prime factors of the base:2 Prime factors of one below the base:7 Prime factors of one above the base:3 Other Prime factors:5 13 15 21 23 27 35 37
Fraction	Prime factors of the denominator	Positional representation	Positional representation	Prime factors of the denominator	Fraction
1/2	2	0.5	0.4	2	1/2
1/3	3	0.3333... =0.3	0.2525... =0.25	3	1/3
1/4	2	0.25	0.2	2	1/4
1/5	5	0.2	0.1463	5	1/5
1/6	2,3	0.16	0.125	2,3	1/6
1/7	7	0.142857	0.1	7	1/7
1/8	2	0.125	0.1	2	1/10
1/9	3	0.1	0.07	3	1/11
1/10	2,5	0.1	0.06314	2,5	1/12
1/11	11	0.09	0.0564272135	13	1/13
1/12	2,3	0.083	0.052	2,3	1/14
1/13	13	0.076923	0.0473	15	1/15
1/14	2,7	0.0714285	0.04	2,7	1/16
1/15	3,5	0.06	0.0421	3,5	1/17
1/16	2	0.0625	0.04	2	1/20
1/17	17	0.0588235294117647	0.03607417	21	1/21
1/18	2,3	0.05	0.034	2,3	1/22
1/19	19	0.052631578947368421	0.032745	23	1/23
1/20	2,5	0.05	0.03146	2,5	1/24
1/21	3,7	0.047619	0.03	3,7	1/25
1/22	2,11	0.045	0.02721350564	2,13	1/26
1/23	23	0.0434782608695652173913	0.02620544131	27	1/27
1/24	2,3	0.0416	0.025	2,3	1/30
1/25	5	0.04	0.02436560507534121727	5	1/31
1/26	2,13	0.0384615	0.02354	2,15	1/32
1/27	3	0.037	0.022755	3	1/33
1/28	2,7	0.03571428	0.02	2,7	1/34
1/29	29	0.0344827586206896551724137931	0.0215173454106475626043236713	35	1/35
1/30	2,3,5	0.03	0.02104	2,3,5	1/36
1/31	31	0.032258064516129	0.02041	37	1/37
1/32	2	0.03125	0.02	2	1/40

Irrational numbers

The table below gives the expansions of some commonirrational numbersin decimal and octal.

Number	Positional representation
	Decimal	Octal
√2(the length of thediagonalof a unitsquare)	1.414213562373095048...	1.3240 4746 3177 1674...
√3(the length of the diagonal of a unitcube)	1.732050807568877293...	1.5666 3656 4130 2312...
√5(the length of thediagonalof a 1×2rectangle)	2.236067977499789696...	2.1706 7363 3457 7224...
φ(phi, thegolden ratio=(1+√5)/2)	1.618033988749894848...	1.4743 3571 5627 7512...
π(pi, the ratio ofcircumferencetodiameterof a circle)	3.141592653589793238462643 383279502884197169399375105...	3.1103 7552 4210 2643...
e(the base of thenatural logarithm)	2.718281828459045235...	2.5576 0521 3050 5355...

See also

• Computer number format– Internal representation of numeric values in a digital computer
• Octal games,a game numbering system used incombinatorial game theory
• Split octal,a 16-bit octal notation used by the Heath Company, DEC and others
• Squawk code,a 12-bit octal representation ofGillham code
• Syllabic octal,an octal representation of 8-bit syllables used by English Electric

External links

• Octomaticsis anumeral systemenabling simple visual calculation in octal.
• Octal converterperforms bidirectional conversions between the octal and decimal system.